Torrefaction and partial pyrolysis to produce fuel pellets with counter current flow of tar

ABSTRACT

The present application discloses a continuous process for the preparation of fuel pellets, said process comprising the steps of a) feeding a feedstock to a torrefaction and partial pyrolysis step at a temperature in the range from 250° C. to 500° C., whereby a solid char and volatile fraction are obtained, said volatile fraction comprising a tar fraction; b) directing the evolved volatile fraction as counter current stream relative to the stream of the feedstock, and at least partially condensing the volatile fraction on the incoming feedstock so as to obtain a tar-rich fraction combined with the incoming feedstock; and c) pelletization of the combined solid char/tar-rich fraction so as to obtain said fuel pellets. Novel fuel pellets are also disclosed.

FIELD OF THE INVENTION

The present invention relates to the field of fuel pellets productionbased on various sources of feedstock, e.g. biomass and waste.

BACKGROUND OF THE INVENTION

Bergman and Kiel, “Torrefaction for Biomass Upgrading”, 14^(th) EuropeanBiomass Conference & Exhibition, 17-21 October 2005, discloses a processof torrefaction of biomass, separation of the volatiles, and cooling ofthe torrified biomass. It is suggested that the torrified biomass mayundergo size reduction and pelletization.

Bergman et al., “Torrefaction for biomass co-firing in existingcoal-fired power stations—“Biocoal””, ECN-C-05-013, Energy researchCentre of the Netherlands (ECN), 2005, discloses a process oftorrefaction of biomass at a temperature of about 280° C., wherein thetorrified biomass is cooled and the torrefaction gas is combusted andused for drying the biomass and as a heat supplement for thetorrefaction process.

Bergman, “Combined torrefaction and pelletisation—The TOP process”,ECN-C-05-073, Energy research Centre of the Netherlands (ECN), 2005,discloses a process of torrefaction of biomass at a temperature of250-300° C. and subsequent pelletization.

In Gilbert et al., “Effect of process parameters on pelletisation ofherbaceous crops”, Fuel 88 (2009), 1491-1497, a study of pelletisationunder various conditions is reported. It was concluded that torrifactionof grass was not an attractive pre-process as the pellets were verybrittle and possessed little mechanical strength and reduced bulkdensity. It was mentioned that heavy pyrolysis oil has a potential foruse as a binding material which can significantly increase the strengthand durability of the pellets.

WO 2010/129988 A1 discloses a process for the preparation of fuelpellet, wherein a feedstock is subjected to torrefaction and/or partialpyrolysis at at temperature in the range from 250 to 500° C., whereby asolid char and a volatile fraction are obtained. The volatile fractionis used for heating of a mixer vessel. The condensed tar maysubsequently be combined with the solid char.

EP 2,287,278 A2 discloses torrefaction of biomass, whereby a solidfraction is directed to a cooler. A rotary valve ensures that thevolatile is not allowed to enter the cooler, but is instead fed to acombustion unit.

US 2009/007484 A1 discloses an apparatus and process for convertingbiomass feed materials into reusable carbonaceous and hydrocarbonproducts. The biomass may be torrified and the volatile fraction iscondensed in one or a series of condensors. The solid material may bepelletized.

SUMMARY OF THE INVENTION

In this text char is defined as biomass or waste with a high organicfraction that has been exposed to a temperature of minimum 200° C.

The present invention provides a process for providing fuel pelletsbased on biomass or waste that can be optimized for use in power plantboilers (grate, fluid bed or suspension fired), district heatingboilers, small pellet stoves, industrial process furnaces, kilns andboilers, small-scale heating devices, and barbeque grills. The pelletsmay be utilized as a global trading product. Process steps to controlpellet heating value density, pellet milling properties, particle sizein pellet and pellet ash properties may be included. From the end-user'spoint of view, the following pellets properties are attractive, namelyi) a high heating value density to minimize transport costs, ii) a highpellet stability and hydrophobic properties of the pellets which makehandling simple, minimized dust problems, and thereby reduce the risk ofself-ignition and provide the option of out-door storage even in wetclimates; iii) the option of easy grinding of the pellets in a mill e.g.a coal mill to obtain a small particle size; and iv) acceptable pelletash properties so that ash deposition, corrosion and flue gas cleaningsequipment interference are minimized and residual product utilization ispossible.

Hence, the present invention provides a continuous process for thepreparation of fuel pellets, said process comprising the steps of

a) feeding a feedstock to a torrefaction and partial pyrolysis step in areactor at a temperature in the range from 250° C. to 500° C., whereby asolid char and a volatile fraction are obtained, said volatile fractioncomprising a tar fraction;

b) directing the evolved volatile fraction as a counter current streamin the reactor relative to the stream of the feedstock, and at leastpartially condensing the volatile fraction on the incoming feedstock soas to obtain a tar-rich fraction combined with the feedstock; and

c) pelletization of the combined solid char and (reheated) tar-richfraction so as to obtain said fuel pellets.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the overall process where tar condensation isarranged counter currently such that tar is condensed already onto theincoming feeding material. Generally, FIG. 1 illustrates the overallprocess for the preparation of fuel pellets. The shown process canprovide torrified pellets with optimal properties for different uses.The process may include several steps as shown in FIG. 1 and the processcan be implemented by use of a variable number of process steps. Thetotal process may include torrefaction, size reduction of torrifiedmaterial, cooling, condensation of tar and separation of gas, possibleaddition of additives and pelletization. The needed heating (for processA in FIG. 1) can possibly be provided by combustion of the evolvedgasses or by another energy source. The heat transfer to the feedstockcan be provided by a hot metal surface, by superheated steam, by bedmaterial e.g. sand, by a flue gas depleted of oxygen or by a materialsuch as ceramic or metal balls or elements of irregular shapes.

FIG. 2 illustrates the implementation of the embodiment of the inventionillustrated in FIG. 1 by use of a screw type reactor. A screw unit and apelletizer are combined, but it should be understood that the inventionalso encompasses where such units are used in sequence without beingbuild together. The feedstock is transported into the pelletizing unitby the screw feeder. In the first part of the screw feeder, thefeedstock is heated to a pre-set temperature to release tar and gas andobtain more fragile properties of the solid char. The residence time isdefined by the rotation velocity of the screw feeder and the dimensionof the screw unit. The volatiles are lead backwards toward the fuelfeeding, the tar is condensed on the incoming feedstock and the gas isreleased near the solid fuel inlet. Finally the combination of thereheated tar and solid char is pelletized.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, the present invention provides a process for thepreparation of fuel pellets in which a feedstock undergoes torrefactionand partial pyrolysis, and wherein produced tar is combined with thein-coming feedstock and the product in form of char and reheated tar arepelletized.

The process can be implemented with a screw type reactor is shown inFIG. 2.

The Feedstock

The process of the present invention may be applied using a wide varietyof feedstock, e.g. a biomass material or waste, including herbaceousbiomass such as straw and grains, wood biomass including hard andsoftwood, as well as in principle all waste types with a significant(>10 wt %) organic fraction, or any mixtures of such feedstock.Preferably, the feedstock has an organic content of at least 15 wt %,such as at least 20 wt %, e.g. at least 40 wt %, or at least 60 wt %.

In one currently preferred embodiment, the feed stock is a biomassmaterial. Many preferred biomass materials have an organic content of atleast 80 wt %, such as at least 90 wt %.

Preferred types of feedstock include straw, grains, hard wood, softwood, and dried sewage sludge. In some embodiments, the feedstock iswood (typically ash content 0.3 to 3 wt %), annual biomass (typicallyash content 3 (or 4) to 10 wt %), or more variable organic wastematerials such as waste wood or dried sewage sludge.

Preferably, the water-content of the feedstock is reduced to 2-15 wt %prior to the torrefaction and partial pyrolysis process of step a)below. Reduction of the water content may be obtained in the firstprocess step by steam drying, heating, compression or centrifugation.

Hence, in one embodiment of the process, step a) (see immediately below)is preceded by a drying step wherein the water-content of the feedstockis reduced to less than 10 wt %.

Step a)

After a possible drying the first step of the process includes acombined torrefaction and partial pyrolysis process (see also Process Ain FIG. 1).

The torrefaction process is carried out by heating the feedstock in asuitable reactor in an inert atmosphere or an atmosphere with less than0.5 Vol % O₂ up to a temperature from 200° C. to 300° C. The atmospheretypically consists of the evolved volatiles, N₂, CO₂, steam or a fluegas depleted of oxygen. The residence time of the feedstock in thereactor at temperatures for torrefaction is typically between 0.5seconds and 2 hours.

Typically, a solid char product yield after torrefaction of 50 to 90 wt% is obtained containing 70-90% of the feedstock heating value. Theresidual product is a volatile fraction (a gas) rich in CO, CO₂ andwater with smaller contents of H₂ and some light hydrocarbons, andpossible small amounts of tar.

At higher temperatures, i.e. from 300° C. to 500° C., the process isdefined as a partial pyrolysis process. The residence time of thefeedstock in the reactor at temperatures for partial pyrolysis istypically from 0.5 second to 1 hour.

The solid char product yield after partial pyrolysis is typically from15 to 85 wt % depending on process conditions (temperature, heatingrate, residence time). The evolved volatiles (i.e. the volatilefraction) contain both a gas and a condensable fraction of tar rich inoxygenated hydrocarbons. The tar yield can be in the range from 2 to 65wt % of the feedstock depending on operation conditions.

It should be understood that the border-line between torrefaction andpartial pyrolysis is somewhat theoretical, because it is found that thevolatile fraction of a torrefaction process already from about 250° C.may comprise tar.

The present invention combines the torrefaction and partial pyrolysis soas to obtain a suitable amount of tar. Hence, the optimaltorrefaction/pyrolysis reactor operation temperature is a compromisebetween two objectives. The temperature shall be sufficiently high toobtain a sufficient yield of tars and thereby to obtain pellets withadequate quality. Also the char yield shall be as high as possible toobtain a maximum of the feedstock energy content transferred to the fuelpellets. Generally, the char yield decrease and the tar yield increasewith increasing reactor temperature. It is not possible to define agenerally applicable optimal reactor temperature for all types offeedstock. However, previously conducted studies indicate that theoptimal temperatures may be in the range of 250 to 500° C. The actualoptimal reactor temperature is dependent on the applied feedstock andreactor type.

However, in some preferred embodiments, the torrefaction and partialpyrolysis involves that the feedstock is subjected to a maximumtemperature in the range from 250° C. to 500° C., such as from 260° C.to 490° C., e.g. from 270° C. to 480° C., or from 280° C. to 475° C., orfrom 290° C. to 470° C., or from 300° C. to 460° C., preferably from310° C. to 450° C., or from 320° C. to 450° C., or from 330° C. to 450°C., or from 340° C. to 450° C., or from 350° C. to 450° C. In otherembodiments, the feedstock is subjected to a temperature in the rangefrom 250° C. to 400° C., such as from 260° C. to 390° C., e.g. from 270°C. to 380° C., or from 280° C. to 360° C., or from 290° C. to 350° C.

The combined torrefaction and partial pyrolysis is typically allowed toproceed for a total period from 2 seconds to 2 hour, such as from 10seconds to 90 minutes, such as from 4 minutes to 90 minutes, or from 6minutes to 70 minutes, e.g. from 8 minutes to 50 minutes.

A possible method to control the quality of the obtained pellets couldbe to use an instrument that determines the amount of condensableproducts in the volatiles fraction. The instrument could determine theamount of condensed material by cooling the volatile fraction to e.g.110° C.

The torrefaction process and the partial pyrolysis process may be run asseparate processes in the same or separate reactors. However,preferably, the processes are run sequentially, e.g. by using atemperature gradient. The processes is implemented (as illustrated inFIG. 1) with counter current flow conditions (as illustrated in FIG. 2and “Preferred embodiment of the process” below).

Hence, in some embodiments, the feedstock is heated for up to 2 hours.Within this embodiment, the exit temperature at completion of thetorrefaction and partial pyrolysis process typically is in the rangefrom 300° C. to 450° C.

A heat source is needed to facilitate the torrefaction and partialpyrolysis process. Heat may be supplied by heat transfer through a metalwall, by an intermediate heat carrier such as sand, ceramic, concrete ormetal balls, steam, CO₂ or by a flue gas nearly depleted of oxygen. Heatcan be generated by using the gas developed in process step a), by usingheat from other processes or by using a separate fuel supply.

A possible size reduction of the char may be performed (see Process B inFIG. 1) in order to obtain a more homogeneous char fraction with reducedparticle size. This could be as a separate process step or integratedwith the torrefaction and/or partial pyrolysis processes.

The output stream from step a) (see Process A (and Process B) in FIG. 1)is a solid char and the volatile fraction (volatile constituents at theexit temperature). The volatile fraction comprises gasses, water andtar. In the present context, “gasses” are defined as the fraction of thevolatiles which is still in the gas phase at 25° C. and 1 atm.

One interesting fraction of the volatile fraction is the tar fraction,which will be discussed further in connection with step b) below.

The torrefaction/pyrolysis process can be implemented by use of a rangeof different reactors, some examples are provided:

-   -   Single or multiple screw reactors. An example is shown in        FIG. 2. The process heat may be provided by external heating of        the screw channel wall, by heating the screw or by injection of        superheated steam.    -   Ball mills or rotary kiln type reactors. The feedstock can be        simultaneously grinded and heated. Heat for the process can be        provided with external heating, steam, heating of metal or        ceramic balls, by other heat carrying materials or by injection        of a sub-stoichiometric hot flue gas. Both the feedstock drying        and torrefaction/pyrolysis units are based on rotary kiln        technology.    -   Fluidized bed reactors, bubbling bed or circulation fluidized        reactors. Heat can be provided by combustion in a separate        secondary bed and hot solids are then mixed with the feedstock        in a primary bed.    -   Fixed or moving bed reactors. The fuel is exposed to a counter        flow of hot flue gas. The exit gas is cooled whereby tar is        provided. Char is removed from the bottom part of the reactor.        Hot flue gas is provided by combustion of a part of the evolved        gas or/and char.

Step b)

An essential feature of the step b) is that the tar-rich fraction iscombined with the incoming feedstock by leading the volatile fractioncounter-stream relative to the stream of the feedstock. Hence, theevolved volatile fraction (gas and tar) is directed backwards (as acounter current stream relative to the stream of the relatively coolerfeedstock), whereby the volatile fraction (including the tar-richfraction) at least partially condenses on the incoming feedstock so asto obtain a tar-rich fraction combined with the incoming feedstock, i.e.the tar-rich fraction thus is allowed to condense on the incomingfeedstock before the incoming feedstock undergoes combined torrefactionand partial pyrolysis (see Process C1 in FIG. 1B; no direct cooling isnormally needed because the feedstock may provide the cooling effect).

The tar-rich fraction is typically condensed when cooling from thetemperature of step a) (i.e. the torrefaction/partial pyrolysistemperature (such as about 350° C.)) to a temperature of 50-150° C. Inmany practical embodiments, no external cooling is necessary, becausethe volatile fraction is directed as a counter current stream relativeto the stream of the feedstock and thereby is cooled by the incomingfeedstock. This is believed to constitute an energy-efficient heating ofthe feedstock and cooling of the volatile fraction.

It is envisaged that a volatile fraction of the tar will re-evaporate(in Process A) and will partly re-condensed (in Process C1) and partlybe converted to a gas, whereas another part of the tar will undergopolymerisation after condensation and heating, so that it will remain onthe solid char after Process A, and will subsequently be cooled (ProcessC2). Hence, it should be understood that a minor part of the volatilefraction may escape to the section of the reactor for cooling of thesolid char, whereby a minor fraction (e.g. typically less than 20%) ofthe tar-rich fraction may condense in Process C2.

Water can be condensed upon cooling to a temperature below 100° C. i.e.below the water drew point temperature. In some embodiments, it isdesirable to allow water to become condensed together with the tar-richfraction in that the presence of water will facilitate the pelletformation (step c)).

In some embodiments, any gasses from the volatile fraction from whichthe tar-rich fraction is condensed, may be combusted so as to provideenergy to any drying of the feedstock or to the torrefaction and partialpyrolysis process. Hence, the evolved gas may be used to provide heatfor, e.g., process step a).

The cooling step, if necessary, can depending on temperature be utilizedfor power or heat production, e.g. by heat exchange with appropriatewater or steam cycles.

For some types of feedstock (typically alkali rich feedstock) and forsome applications of the fuel pellets, it may be advantageous to combineadditives (see Process D in FIGS. 1 and 1B) with the solid char (and thetar), which in a combustion process can bind alkali metals or otherspecies and make them less harmful.

Hence, in some embodiments, it is—for the purpose of making optimalpellets for different combustion and gasification units—advantageousthat the pellets are formulated by addition of additives (see Process Din FIG. 1) prior to pelletization. The additives may be clay minerals,lime stone, bleaching soil, sewage sludge or other waste products.Generally materials containing more than 5 wt % of one or several of thefollowing elements may be used: S, P, Al, Si and Ca. The additives areprovided so as to modify the properties of the pellets, e.g. such thatash deposition and corrosion problems during pellet combustion areminimized. Additives promoting/catalyzing the tar curing process mayalso be added.

Examples of pellets formulations may include:

A. Prevention of slagging in the bottom part of small scale pelletstoves. Often melting of bottom ash appears in the bottom part of pelletstoves whereby fuel feeding is disturbed. An addition of calciumcontaining species may increase the melting temperature of the producedbottom ash. Addition of limestone to obtain a molar ratio of Ca/K morethan 2 in the fuel pellets will often be sufficiently to prevent bottomash slagging.

B. When biomass based pellets are used in large dust fired power plantboilers problems with severe deposit formation on the super heaters maybe observed. This makes problems both with accumulation of deposits andcorrosion of super-heater tubes. Addition of sufficiently amounts ofminerals rich in Si and Al may remedy those problems. Obtaining a fuelpellet with a molar ratio of more than 2.5 of (Si+Al)/(K+Na) maysignificantly reduce problems.

Any additives may be combined with the solid char before, in combinationwith, or after combination of the solid char with the tar-rich fraction.In some embodiments, the additives may even be fed together with thefeedstock.

Step c)

In step c) of the process (see Process E in FIG. 1), the combination ofthe solid char (preferably in particulate form after grinding), thecondensed and (reheated) tar-rich fraction (see above) and any additives(see step b) is pelletised.

Keeping of the material at a temperature in the range from 50° C. to100° C. of the pelletizing process may increase pellet stability andhardness.

The pelletizing is conducted using conventional equipment, e.g. anAndritz sprount pellet mill, using conventional conditions.

The pelletizing may be followed by a curing step in order to harden thepellets, e.g. by curing the tar.

Preferably step a) and step b) of the process are run as a continuousprocess. In some interesting embodiments hereof, step a), step b) andstep c) of the process are run as a continuous process.

Preferred Embodiment

Hence, the present invention also provides a continuous process for thepreparation of fuel pellets, said process comprising the steps of

a. feeding a feedstock, preferably a biomass selected from wood, to atorrefaction and partial pyrolysis step at a temperature in the rangefrom 250° C. to 500° C., such as 250-400° C., e.g. 300-350° C., wherebya solid char and a volatile fraction are obtained, said volatilefraction comprising a tar fraction;

b. directing the evolved volatile fraction counter current streamrelative to the stream of the feedstock, and at least partiallycondensing the volatile fraction on the incoming feedstock so as toobtain a tar-rich condensed fraction combined with the incomingfeedstock; and

c. pelletization of the combined solid char and (reheated)tar-richfraction so as to obtain said fuel pellets.

The product can advantageously be stored and transported with highstability and the pellets can be used as fuel in a pulverized firedpower plant boiler.

Pellets

The primary demands for an adequate pellet quality is a pellet that ishydrophobic and does not fragment significantly during transportation.Hence, the pellets should also have suitable mechanical strength, e.g.defined as the tensile strength thereof. The tensile strength can bemeasured using a tensometer for compression of a pellet in the radialdirection, cf. the method described by da Rocha SSHF, “Mechanicalevaluation for the quality control of biomass pellets and briquettes.In: Proceedings of the second world conference on pellets, Jönköping,Sweden; 2006, 183-187.

It appears that useful pellets preferably have a tensile strength of atleast 100 kPa, such as at least 200 kPa, e.g. at least 300 kPa. Veryattractive pellets are those having a tensile strength of at least 400kPa, such as at least 500 kPa, or at least 600 kPa, or at least 700 kPa.

Hence, it is believed that the pellets obtained by the above process arenovel as such. Hence, the present invention also provides a fuel pelletcomprising a solid char, tar, and, optionally, one or more additives,said solid char and said tar being obtained by torrefaction and partialpyrolysis of a feedstock at a temperature from 250° C. to 500° C.Preferably, the pellet has a tensile strength of at least 100 kPa.

The pellets prepared according to the invention can be grinded with lowenergy consumption and is thereby optimal to use in suspension firedboilers. Moreover, the pellets can be stored under out-door conditionson moist regions of the world, e.g. in the Scandinavian countries.

Use of Pellets

The pellets prepared according to the invention can be provided to anational or an international market with end-uses in: power plantboilers (grate, fluid bed or suspension fired), district heatingboilers, small pellet stoves, industrial process furnaces, kilns andboilers, small-scale heating devices, and barbeque grills.

1. A continuous process for the preparation of fuel pellets, saidprocess comprising the steps of a. feeding a feedstock to a torrefactionand partial pyrolysis step in a reactor at a temperature in the rangefrom 250° C. to 500° C., whereby a solid char and a volatile fractionare obtained, said volatile fraction comprising a tar fraction; b.directing the evolved volatile fraction as a counter current stream inthe reactor relative to the stream of the incoming feedstock, and atleast partially condensing the volatile fraction on the incomingfeedstock so as to obtain a tar-rich fraction combined with thefeedstock; and c. pelletization of the combined solid char and(reheated) tar-rich fraction so as to obtain said fuel pellets.
 2. Theprocess according to claim 1, wherein the gas is released near the solidfeedstock inlet.
 3. The process according to claim 1, wherein theprocess further comprises size reduction of the solid char, wherein thesize reduction takes place as a part of step a) and/or immediatelysubsequent to step a). 4-6. (canceled)
 7. The process according to claim2, wherein the process further comprises size reduction of the solidchar, wherein the size reduction takes place as a part of step a) and/orimmediately subsequent to step a).
 8. The process according to claim 1,wherein the water in an amount of up to 0-15 wt % by weight of the solidchar is combined with the solid char in step c).
 9. The processaccording to claim 2, wherein the water in an amount of up to 0-15 wt %by weight of the solid char is combined with the solid char in step c).10. The process according to claim 3, wherein the water in an amount ofup to 0-15 wt % by weight of the solid char is combined with the solidchar in step c).
 11. The process according to claim 7, wherein the waterin the amount of up to 0-15 wt % by weight of the solid char is combinedwith the solid char in step c).
 12. The process according to claim 1,wherein additives are provided in the pellets so as to modify theproperties of the pellets.
 13. The process according to claim 2, whereinadditives are provided in the pellets so as to modify the properties ofthe pellets.
 14. The process according to claim 3, wherein additives areprovided in the pellets so as to modify the properties of the pellets.15. The process according to claim 8, wherein additives are provided inthe pellets so as to modify the properties of the pellets.
 16. A fuelpellet comprising a solid char, a tar, and, optionally, one or moreadditives, said solid char and tar being obtained by torrefaction andpartial pyrolysis of a feedstock at a temperature from 250° C. to 500°C., wherein said pellet has a tensile strength of at least 100 kPa.